1. Field of the Invention
The present invention relates generally to a method and apparatus for producing a molten metal from powder state ore. More specifically, the invention relates to a process for smelting powder state ore by utilizing a shaft furnace.
2. Description of the Background Art
In the recent years, the ratio of powder state ore as a material for producing metal has increased. Especially, according to advances in ore dressing techniques, ratio of the powder state ore is expected to further increase. In this viewpoint, there has been proposed a technique for smelting the powder state ore in a shaft furnace filled with carbon containing reducing material, such as coal or coke.
Smelting process utilizing a shaft furnace has been developed and proposed. In the known process, solid state carbon containing reducing material is charged through the top of the furnace. To the furnace filled with the carbon containing reducing material, oxygen containing gas is blown through tuyeres in order to form a fluidized bed at the upper section of the furnace. Below the fluidized bed, the reduction material filled section is formed. The powder state ore is also blown into the furnace to perform the smelting operation.
By blowing the oxygen containing gas. substantially high temperature race ways are formed around the tuyeres. The powder state ore blown into the furnace through the tuyeres is instantly molten in the race ways. Molten ore flows down through the reducing material filled section or otherwise is fluidized with the reducing material in the fluidized bed. During fluidization, reduction of the ore progresses to refine it. According to the progress of reduction, the density of the molten ore gradually increases. At the same time, the reduced ore undergoes sticking and melting to gradually increase the grain size. The increased grain size of the ore moves down through the reducing material filled section. During downward travel, reduction of the ore is completed. At the same time, the temperature of the ore is increased to the tapping temperature. On the other hand, during the aforementioned reduction process, the ore absorbs metalloids such as Si and Mn. Furthermore, during the reduction process, separation of the metal component and the slag component occurs so that molten metal and slag are separately collected in the bottom of the furnace.
Such a smelting technique is effective for efficiently producing molten metal from the powder state ore. However, the prior proposed technique has a drawback in that the reducing material to be used has to have a grain size large enough so as not to be blown away by the gas flow. The grain size of the reducing material may be variable depending upon the gas flow velocity in the furnace, which varies as flow velocity varies depending upon the temperature in the furnace, pressure, gas flow amount and so forth. The grain size of the reducing material at the borderline between being blown away and not being blown away in relation to the gas flow velocity will be hereafter referred to as "gas flow velocity corresponding grain size". In the practical operation, in consideration of fluctuation of the gas flow velocity, the grain size of the reducing material to be charged in the shaft furnace is selected to be n-times greater than the gas flow velocity corresponding grain size. In such case, reducing material having smaller grain size than the gas flow velocity corresponding grain size will never be used. On the other hand, even when the reducing material which has smaller grain size than the gas flow velocity corresponding grain size, such small grain size reducing material may be easily blown away with the exhausting gas. This apparently increases the cost for producing the molten metal.
On the other hand, temperature and composition of the molten metal are variable depending upon the temperature in the reducing material filled section of the furnace. Therefore, in order to stably produce high quality molten metal, it is essential to control the temperature of the reducing material filled section.
The Japanese Patent First (unexamined) Publication (Tokkai) Showa 62-56537 discloses a method for producing molten metal from powder state ore by forming the fluidized bed of the reducing material and the reducing material filled section in the shaft furnace. However, the disclosed system cannot control the temperature of the reducing material filled section. Therefore, it was not possible to stably perform production of the molten metal and maintain the quality of the produced molten metal at satisfactorily high level.